1. Field of the Invention
The present invention relates, in general, to methods and systems of fan control, and more particularly, to a design for utilizing spread-spectrum techniques to minimize the amplitude of mechanical and/or acoustical vibration coupled to a chassis of an electronic device at any given frequency.
2. Relevant Background
Computer systems such as rack server systems contain numerous heat-generating electronic components (e.g., power supplies, hard disk drives, Peripheral Component Interconnect (“PCI”) or PCI Express cards). Generally, the excess heat that these components generate is actively dissipated through the use of one or more cooling fans that circulate air throughout the inside of a housing or chassis for the electronic system. The circulating air carries away the excess heat within the chassis, thereby reducing internal heat build-up that can lead to misoperation and long-term damage to the components.
Current fan control technologies commonly employ pulse width modulation (“PWM”) control methods to control the speed of direct current (“DC”) cooling fan motors. In response to an input voltage, a PWM signal generator generates a PWM control signal, which may be represented as a periodic rectangular wave having an alternating sequence of on-time and off-time. The fraction of time that the signal is active equates to the duty cycle of the PWM signal. For example, where the on-time pulse duration (t) is 1 second and the period (T) of the PWM signal is 2 seconds, the duty cycle is 50 percent.
Generally, conventional PWM algorithms use a constant control variable to set an input voltage to a PWM generator. In turn, the PWM generator outputs a PWM control signal having a constant duty cycle to the controlled cooling fan or fans. The constant control variable is often a function of temperature (e.g., the temperature within the chassis or of specific components within the system) and/or other measurable physical states of the system. In this regard, a fan speed controller may incorporate a fan speed sensor (e.g., a tachometer) as well as a thermal monitor (e.g., a diode connected transistor). The fan speed controller (e.g., an integrated circuit including the PWM generator) adjusts the control variable in response to a feedback temperature signal generated by the thermal monitor in order to increase or decrease the rotational speed of the cooling fan(s) as necessary to maintain a desired physical state (e.g., a thermal state) within the system. Thus, the PWM control signal for one or more controlled fans is either held constant, thereby maintaining a constant duty cycle of the PWM control signal, or is incrementally adjusted only as necessary to maintain the desired physical state within the system. In other words, when the thermal monitor indicates a desired temperature, the control variable—and thus the duty cycle of the resulting PWM control signal—remains static. This constant duty cycle results in a constant rotational speed of the controlled fan or fans.
The advancement of the computer and electronics industry demands ever-increasing numbers of heat-generating electronic components to be integrated into electronic devices such as server systems. As a result, air-cooled server systems continually command more cooling fans that require more power to accommodate faster rotational speeds. These increased airflow needs lead to increased mechanical and/or acoustical vibration or noise that is coupled to a chassis of the server system at any given rotational frequency.